Distance sensing and other distance measurement related applications have been accomplished in the prior art optically utilizing the principles of triangulation. A simplified example of a utilization of the principle is illustrated in FIG. 1. A light source comprising a light emitting diode LED 10 emits a pulsed light beam that is collimated along its optical axis and directed toward a target or object T. With the target T at the position X1 from the LED 10 a portion of the pulsed light beam is reflected back to a photoreceiver means such as a position sensitive detector (PSD) 12 well known in the prior art. The reflected light beam strikes the surface of the position sensitive detector 12 at the position Y1. The PSD 12 converts the photon light energy striking the PSD into two electrical current signals I1 and I2 at two of its output terminals. The current signals I1 and I2 contain information relative to the position where the reflected light beam impinges upon the surface of the PSD. Accordingly, the current signals I1 and I2 contain information relating to the distance X1. When the position of the reflected light beam moves in a vertical direction as illustrated in FIG. 1, the difference between the signals I1 and I2 changes. With the target at the position X2, the position of the reflected light beam on the PSD is shown at Y2. The displacement between Y1 and Y2 corresponds to the difference between X1 and X2. It is known in the prior art to electronically process the changes of the current signals I1 and I2 to generate distance measurement related signals. The position Y of the light beam on the surface of the PSD satisfies the following equation: ##EQU1##
A common method to extract distance information from the currents I1 and I2 is to use this equation. To accomplish the signal division, straightforward approaches such as microprocessors with analog to digital (A/D) converters, analog dividers or logarithmic operational amplifier circuits are used. The prior art system design using a microprocessor is straightforward incorporating off the shelf electronic circuitry. The mathematical operations of division of the signals are performed with the microprocessor. The circuitry required for this implementation are microprocessors with analog to digital conversion. In the prior art analog devices have also been used to accomplish the same division function.
The currents I1 and I2 are usually small and the photon density or intensity of light striking the surface of the PSD varies with distance. Thus, complex electronic circuits are required for current amplifications and the electronic signal processing. Two factors in an optical triangulation system make the signal processing difficult. The first is the change of the magnitude of the currents I1 and I2 as a function of the distance between the LED and the target. The other is the surface reflectivity of the target. For example, with a low reflectivity target the photon density or light intensity impinging upon the surface of the PSD will be less than a high reflectivity target. The light intensity striking the PSD decreases dramatically as the target moves farther away from the LED and is approximately inversely proportional to the square of the distance between the PSD and the target at relatively far distances. The light intensity striking the PSD is never a constant value when the target moves along the optical axis. For a given distance between the PSD and the target, the reflected light striking the PSD is likely to vary according to the color of the target which affects the surface reflectivity.
Therefore the two current signals I1 and I2 contain more than just distance information. The signals also include the effective surface reflectivity from a target and light intensity changes at the PSD due to distance changes. Therefore, the simple difference between I1 and I2 is no longer the direct information of distance measurement. Because of the mixed information in the signals, the signal processing for extracting the distance measurement information is difficult.
Because of the change in the received light energy as a function of distance and the reflectivity of the target, the dynamic range of the current signals is large. The large dynamic range requires a high resolution A/D converter. Usually a high resolution A/D converter is relatively costly and has a slow response time. The high dynamic range also impacts amplifier design of the processing circuitry since the circuitry must be relatively noise free and operate with very low and very high levels of current. The use of analog dividers costs even more because of the requirement for the dynamic range characteristics and noise immunity. Analog dividers are, by their very nature, high gain devices where noise and stability are difficult to control.
Another prior art technique used instead of directly dividing the current signals from the PSD is the use of logarithmic operational amplifiers. These devices translate the dividing function into a straightforward subtraction method. But the necessary components for the logarithmic approach suffer from the linearity for the large dynamic range and performance drift with temperature variation.
Another approach to the signal processing is disclosed in U.S. Pat. No. 5,055,664. This patent discloses a ramp generator technique to normalize the denominator in the position equation. The technique disclosed in the '664 patent is an open loop solution requiring the concurrent monitoring of both electrical currents from the PSD. Response speed is limited with this approach.
Another prior art technique is disclosed in Japanese Patent laid-open No. 13412-1989. The technique disclosed in this prior art patent is to maintain the sum of I1 and I2 constant to normalize the denominator of the position equation noted above.
The present invention addresses the shortcomings of the prior art. The present invention eliminates the need for a microprocessor, analog divider or a logarithmic operational amplifier. It is a technique wherein one of the two current signals generated by the PSD is kept constant by regulating the intensity of the light emitted from the LED. The other current then contains the distance measurement information.